The present invention relates to an automatic document feeder with a position compensating device for precisely locating a sheet, document or manuscript onto a predetermined position on a platen.
A document feeder has been known, wherein a transfer belt is located to cover a platen for an image printing apparatus, such as a copy machine, and a sheet or manuscript is transferred on the platen by actuation of the transfer belt.
In this machine, it is important that the sheet is stopped precisely at a predetermined copy position. In order to precisely locate the sheet at the copy position, a document feeder as disclosed in Japanese patent publication (examined) No. 3-56615 compensates timing for generating a stop signal for the transfer belt, wherein an error until the sheet reaches the copy position is obtained based on the distance that the sheet is transferred only by the transfer belt and a memorized slipping amount for a predetermined distance between the belt and the sheet, and pulses for a motor for actuating the transfer belt are adjusted based on the pulses of the error.
However, even if the timing for stopping the transfer belt is adjusted based on the slipping amount for the predetermined distance and the transfer distance of the sheet, if the sheet can not be stopped immediately by stopping the motor for actually actuating the transfer belt at the same time of receiving the stop signal, such compensation is insufficient.
Namely, in the conventional compensating method, the compensation for an overrun amount of the motor for the transfer belt, i.e. the distance that the sheet is actually stopped as soon as the belt stop signal is outputted, is insufficient. Thus, it is impossible to sufficiently adjust the stopping position.
Also, it is difficult to keep the sheet transfer speed constant before the stopping position, since the sheet size and the sheet transfer mode are not constant. Since the sheet transfer speed is different, the overrun amount is not constant, so that it is difficult to improve stopping accuracy of the sheet.
Therefore, an object of the invention is to provide an automatic document feeder, wherein a sheet or document can be stopped at a predetermined position with a very little tolerance.
Another object of the invention is to provide an automatic document feeder as stated above, wherein a stopping position can be controlled precisely regardless the size or copy mode of the sheet.
Further objects and advantages of the invention will be apparent from the following description of the invention.